1. Field of the Invention
The present invention relates to a catalytic combustion type gas sensor for sensing leakage of various kinds of gasses and, more particularly, to a catalytic combustion type gas sensor configured to be able to self-diagnose deterioration or operation abnormality due to use for a long time.
2. Description of the Related Art
Conventionally, a catalytic combustion type gas sensor has been known as a sensor for sensing combustible gasses such as a hydrogen gas, a methane gas, and so on. The catalytic combustion type gas sensor employs a sensing element, in which a heater coil is embedded in a thermal conduction layer (catalyst carrier) having an oxidation catalyst covering its surface or carrying the catalyst for causing a gas to be sensed to combust (oxidize) by contact, and energizes the heater coil of the sensing element to heat it to a predetermined temperature, so that when a combustible gas comes into contact with the oxidation catalyst and causes combustion, the combustion raises the temperature to change the resistance value of the heater coil. The sensor detects the change as a voltage to sense the presence of the combustible gas (see, for example, JP H3-162658A).
The above patent document also discloses a gas sensing apparatus configured such that, to compensate the effect by a change in ambient temperature, a compensation element is connected to the sensing element in series and connected in parallel with a series circuit in which two resistors are connected in series to constitute a Wheatstone bridge circuit, and a direct-current voltage is applied across the parallel circuit to detect the voltage between the connection point of the sensing element and the compensation element and the connection point of the two resistors. As the compensation element in this case, a compensation element is used which has a heater coil having the same electric property as that of the sensing element, embedded in a thermal conduction layer which is not covered with or does not hold the oxidation catalyst.
Such a catalytic combustion type gas sensor is widely used in various kinds of household and industrial equipment using a combustible gas and as a gas leakage sensing apparatus in a room having those kinds of equipment installed.
However, the catalytic combustion type gas sensor has a problem. The problem is that since the sensor is to be used for a long time, deterioration of the sensing element or the like and malfunction of circuits may occur, but such an abnormal state, if occurring, is not known because an actual gas leakage rarely occurs.
For this reason, conventionally, a gas to be sensed has been actually used to carry out an inspection of the sensing operation, or gas sensors in two lines have been provided for inspection and comparison or to increase safety.
However, periodical inspection using an actual gas takes a lot of trouble, and it is difficult, in particular, for an inspector to visit and inspect individually the gas sensor provided in a small-size household system. Another problem is that provision of the gas sensors in two lines leads to an increased cost, and therefore it is difficult to employ the gas sensors in two lines in the small-size system. Even if the gas sensors in two lines are provided, both of the sensors might possibly malfunction.